Independent Relationship between Amyloid Precursor Protein (APP) Dimerization and γ-Secretase Processivity

Altered production of beta-amyloid (A beta) from the amyloid precursor protein (APP) is closely associated with Alzheimer's disease (AD). APP has a number of homo- and hetero-dimerizing domains, and studies have suggested that dimerization of beta-secretase derived APP carboxyl terminal fragment (CTF beta, C99) impairs processive cleavage by gamma-secretase increasing production of long A beta s (e. g., A beta 1-42, 43). Other studies report that APP CTF beta dimers are not gamma-secretase substrates. We revisited this issue due to observations made with an artificial APP mutant referred to as 3xK-APP, which contains three lysine residues at the border of the APP ectodomain and transmembrane domain (TMD). This mutant, which dramatically increases production of long A beta, was found to form SDS-stable APP dimers, once again suggesting a mechanistic link between dimerization and increased production of long A beta. To further evaluate how multimerization of substrate affects both initial gamma-secretase cleavage and subsequent processivity, we generated recombinant wild type- (WT) and 3xK-C100 substrates, isolated monomeric, dimeric and trimeric forms of these proteins, and evaluated both epsilon-cleavage site utilization and A beta production. These show that multimerization significantly impedes gamma-secretase cleavage, irrespective of substrate sequence. Further, the monomeric form of the 3xK-C100 mutant increased long A beta production without altering the initial epsilon-cleavage utilization. These data confirm and extend previous studies showing that dimeric substrates are not efficient gamma-secretase substrates, and demonstrate that primary sequence determinants within APP substrate alter gamma-secretase processivity.